The orange bullseye is an image of two galaxies. One, at the centre, has stretched the image of the other into a ring — the first complete ‘Einstein ring’ ever seen. This is just one of many gravitational lenses collected in recent years, by a programme that aims to answer fundamental questions about the Universe's geometry and composition.

Einstein's theory of General Relativity predicts that light rays are curved by gravitational fields. The first confirmation of this effect came in 1919, when the Hyades star cluster was observed slightly out of place during a solar eclipse — a weak example of gravitational lensing. More interesting lenses occur where one distant galaxy or quasar appears to us in several distorted images, around the position of an intervening galaxy or galaxy cluster. More than 20 are known, including some partial Einstein rings, where the lensed and the lensing objects are almost perfectly aligned.

Prompted by observations from the MERLIN array of radio telescopes, the Hubble Space Telescope has imaged this complete Einstein ring (L. J. King et al. Mon. Not. R. Astron. Soc. 295, L41-L44; 1998). The radio image, shown below Hubble's infrared image, isn't a complete ring because the radio-emitting part of the lensed galaxy is a little off-centre.

So how do lenses reveal the geometry of the Universe? It is a matter of statistics. The redshift distribution of lensed objects tells us what proportion of our lines of sight to distant regions are blocked by intervening galaxies. That is a function of the geometry of space-time, which in turn depends on the amount of matter present, and on the energy density of empty space — the ‘cosmological constant’, which Einstein introduced and then abandoned, but which is beginning to come into vogue again. By the end of this year, the team working with MERLIN hope to put the strictest limits yet on the cosmological constant.